Optical scanning systems are used in various types of machines and the invention to be described herein is to a system which can be used in many of these different machines. For illustrative purposes, the invention will be described in the context of an electrophotographic printing machine.
In electrophotographic printing machines, prints are produced by creating an electrostatic representation of the print on a photoreceptive surface, developing the image and then fusing the image to print material. In machines which utilize plain bond paper or other image receiving material not coated with photoreceptive material, the electrophotographic process is of the transfer type where photoreceptive material is placed around a rotating drum or arranged as a belt to be driven by a system of rollers. In the typical transfer process, photoreceptive material is passed under a stationary charge generating station to place a relatively uniform electrostatic charge, usually several hundred volts, across the entirety of the photoreceptive surface. Next, the photoreceptor is moved to an imaging station where it receives light rays generated by a light source. In electrophotographic printing machines such as the IBM 6670 Information Distributor, light rays are used to discharge the photoreceptive material in white or background areas to relatively low levels while areas which are desired to print out as dark areas continue to carry high voltage levels after the exposure. In that manner, the photoreceptive material is caused to bear a charge pattern which corresponds to the printing, shading, etc. which is desired.
Production of the image on the photoreceptive surface can be produced by a scanning optical beam where the desired characters are produced by driving a light generating source from information held in digital memory. The generating source may be a laser gun, an array of light-emitting diodes, light modulators, etc. which direct light rays to the photoreceptor and cause it to bear the desired charge pattern.
An example of a light scanning and printing system such as is employed in the IBM 6670 Information Distributor is disclosed in U.S. Pat. No. 3,750,189. In that system, a laser beam is directed through a collimating lens system and focused as a line on a rotating mirror. The reflected beam is passed through a combination of a toroidal and a spherical lens to focus the line image on the final image plane, that is, the photoreceptor. The shape of the focused beam on the photoreceptor is designed to be slightly elliptical in order to compensate for the different image forming properties of the optical system in the scan and non-scan directions.
The type of light source provided in the IBM 6670 Information Distributor for use with the optical system of the above-referenced patent is a helium neon generating source or a similar type source which provides a continuous laser beam modulated to carry the digital information desired for reproduction. In the current invention, an optical system is provided which is designed for use with semiconductor laser sources where significantly different problems are encountered and which require solution in order to accurately and economically produce a satisfactory image.
The special problems associated with semiconductor laser generating sources include changes in the wavelength being produced by the laser chip as the chip ages or as the chip experiences heat variations in the machine environment in which it is located. Another problem is a difference in wavelength produced from one laser chip to another. Because of these factors, the inventors herein describe an optical system which is designed to accommodate changes in wavelength produced by the chip.
Another significant problem which must be addressed is variation in the divergence angle of the laser beam which is produced by different chips and which is produced in different planes by the same chip. In addition, an optical system should provide depth of focus to eliminate various problems including astigmatism, and counteract tilt variations in the surface of the rotating mirror. The system of this invention also accommodates these factors.